NiO-TiO₂ p-n Heterojunction for Solar Hydrogen Generation

Round 1

Reviewer 1 Report

The present manuscript reports on the effect of different preparation methods on photocatalytic hydrogen production over NiO-TiO₂ composites. Experimental conditions and kinetic studies are well described and properly carried out. However, the interpretation of the characterization data is approximate or incorrect and no information or suggestions are provided regarding the mechanism of action. Therefore, the manuscript may be accepted for publication after major revision. See details here below.

1. No evidence of the presence of NiO species is provided for samples S1 and S2. XRD patterns shown in Figures 2c and 3a do not present any peak related to Ni-containing species. Moreover, the comparison of Figure 3e and 3f clearly shows that Ni and O signals over sample S2 are not correlated: where Ni signal is intense no appreciable signals of O are present, differently Ti-rich zones present high O signals. Thus, it can be assumed that metallic Ni or only partially oxidized Ni (e.g., core-shell structures) is present in sample S2. Please completely revise the interpretation of TEM images at lines 132-134. Finally, clear evidence of the presence of NiO in samples S1 and S2 must be provided to state that the photocatalytic material is NiO-TiO₂.

 

2. The only one photocatalyst for which the presence of NiO at the TiO₂ surface it is unambiguously determined by XRD and HAADF-STEM is sample S3. However, this sample shows the lowest photocatalytic activity. Please provide an explanation to this different behavior compared to samples S1 and S2.

 

3. A hypothesis of the mechanisms of action is missing. Please add this information to the manuscript.

 

4. Figure 5 shows that the H₂ evolution rates are not constant but they increase during the experiments under illumination for all NiO-TiO₂ This may be an indication of a self-healing process, i.e. the in-situ reduction of oxidized Ni species to metallic Ni, as very recently demonstrated by operando XAS measurements (see DOI: 10.1021/acscatal.0c01373). The presence of metallic Ni is linked to improved photocatalytic performances. Please add this discussion with the provided reference to the text.

 

5. (Figure 5b) Please provide information regarding pure NiO used as reference material (e.g. in the experimental section).

 

6. Labels “S2-30%” and “S3-30%” at lines 207 and 213, respectively, are wrong. Please revise them.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Hydrogen production from water using energy from sunlight is one of the cleanest means of energy generation. The search for efficient catalysts that can drive this reaction are still being sought. So this work on NiO-TiO₂ p-n heterojunction is definitely interesting and would attract the readership of authors following catalysts. However, there are few issues that need to be addressed by the authors before the manuscript can be published:

1. It is known that combining n-type semiconductors with a p-type semiconductor is a good strategy to efficiently separate charges. This was also indicated in the abstract (Line 12 to 14) of this paper. However, from the experimental section, it was indicated that methanol was used as a hole scavenger (reaction mixture contained 50 mL water and 50 mL methanol). How did the authors confirm their hypothesis that the p-n heterojunction was effective in separating charges when they used an additional hole scavenger (up to 50%) for the testing? Was the test conducted also in pure water without methanol for comparison?
2. The performance reported for the p-n heterojunction materials in this study looks very promising, what is the potential of using this p-n heterojunctions on a commercial/larger scale considering the cost of NiO and energetic input of material preparation (180 °C for 12 h, followed by calcination at 600 °C, 2h)?
3. In Figure 1, Strategy 3, the description says the NiO precursor was dissolved in acetic acid, but the scheme shows ethanol? At what temperature was the reaction mixture heated until solid was obtained? This should be indicated in the manuscript.
4. In the experimental description of Strategy 2 and 3, it was mentioned that TiO₂ synthesized in Strategy 1 was used. The TiO₂ from Strategy 1 was prepared in two steps, first by (i) hydrothermal treatment of the precursor and followed by (ii) calcination. Which of the TiO₂ from strategy 1 was used in Strategy 2 and 3? Was it the as-prepared TiO₂ produced after hydrothermal treatment or TiO₂ after hydrothermal + calcination? This is not clear; the authors should clarify for interested readers.
5. The reference list should be formatted with appropriate subscripts for e.g. TiO2, H2, O2 etc. should be TiO₂, H₂, O₂
6. The English need slight polishing for e.g.
   3. Section 3.2.1 “Then 2 mL ……..was dropwise added…. Should rather be …..added dropwise……
   4. In the introduction, Line 26, “environmental pollution” requires no definite article before it. 

Author Response

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Reviewer 3 Report

The manuscript entitled “NiO-TiO₂ p-n Heterojunction for Solar Hydrogen Generation” is original and interesting.

However, the reviewer suggests minor revision:

• Line 205-207 – “The final NiO-TiO₂ samples were obtained after calcined at 600°C for 2h and the theoretical contents of 10% and 20% of Ni in mole were labelled as S2-10% and S2-30%, respectively.” - S3-30% or S2-20%?
• Line 211-213 – “The solid was calcined at 600°C for 2h  and the theoretical contents of 10% and 20% of Ni in mole were labelled as S3-10% and S3-30%, respectively.” - S3-30% or S3-20%?
• Figure 5 (c) - In the caption of Figure 5 (c) the authors write: “cycling tests of S1-20%, S2-20% and S3-10%”, but in this figure cycling test for S3-20% is depicted.
• Line 104-105 – “A nanocrystal with a size of around 20 nm can be clearly observed for S1-10% (Fig. 2a).“ – nanocrystal or nanocrystals?

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

All issues raised by the reviewer have been addressed. Thus, the manuscript can be accepted in its present form.